Advanced technological uses for wire and cable has imposed many new requirements upon traditional wire and cable specifications and functions. In high power shielded cable environments with corona effects for example, there is a need for shielded cable that can operate more efficiently and effectively at higher frequencies.
Shielded wire and cable is often required to meet stringent shielding specifications when utilized in missiles or aircraft. Such wire and cable articles often have to operate in radiation and electrical interference fields without compromising the on-board electronics.
Presently, wire and cables are shielded electrically by braiding wire mesh shields disposed about the primary wire core and insulation. This shielding is meant to prevent RFI and EMI disturbances from influencing the signals in the cable.
As the advanced technology requirements impose greater stringency in shielding frequency specifications, these previously functional braided articles become unacceptable. Shielding leakages occur in these conventional cables by virtue of the looseness by which the wire mesh is braided, leaving holes in the shield web. In addition, the stiffness of the metal wire used in braiding makes it difficult to conform the mesh to the insulation core surfaces, leaving small gaps. Such gaps limit the frequency range in which the cable or wire can be operationally effective.
In high power environments, shorts pose a particular hazard when utilizing shielded cable.
The present invention has resolved the aforementioned problems by the development of a new type of shielded wire and cable article. The new article of this invention contemplates the use of shielding composed of conductive polymer tape wraps or an extruded conductive polymer layer that is utilized in conjunction with the braided mesh shield. The conductive polymer materials provide a homogeneous layer that complements the standard metal wire mesh braiding. The homogeneity of the conductive polymer layer reduces interference leakage and contributes to a higher shielding frequency range capability.
Generally, the conductive polymer layer is combined above or below conventional braided mesh shields. The conductive polymer can be applied as a jacket layer over the conventional wire mesh shield layer. The two combined shield layers will improve the shielding effectiveness in the EMI region at frequencies higher than 10 MHz. The transfer impedance of the inventive cable can range from approximately 0.08 ohm/meter to about 0.5 ohm/meter at 1 GHz.